The present invention relates to a method for fabricating a nitride semiconductor in which the density of a p-type dopant is positively increased, a method for fabricating a nitride semiconductor device, and a nitride semiconductor device fabricated by this method.
Prior art techniques of doping a nitride semiconductor device with a p-type dopant, in particular, magnesium (Mg) will be described.
In the first prior art (Japanese Journal of Applied Physics, 38, L1012, 1999), a superlattice (SL) layer having a cycle of 36 nm is disclosed for use as a p-type cladding layer paired with an n-type cladding layer to sandwich an active layer in the direction vertical to a substrate and confine light generated from the active layer. Each cycle of the superlattice layer is composed of an aluminum gallium nitride (Al0.15Ga0.85N) layer having a thickness of 24 nm and a gallium nitride (GaN) layer having a thickness of about 12 nm, for example. In this disclosure, the cycle of the superlattice layer is in the range of 9 nm to 100 nm.
Doping of the p-type cladding layer with magnesium (Mg) is performed uniformly over the entire superlattice layer. There is another disclosure reporting doping of either the AlGaN layers or the GaN layers. In either case, doping is uniform in each layer of the AlGaN layers and/or the GaN layers. This p-type cladding layer is formed on a substrate in a following manner. That is, using decompressed metal-organic vapor phase epitaxy (MOVPE) under a growth pressure of 300 Torr (1 Torr=133.322 Pa), a buffer layer made of aluminum nitride (AlN) is grown on a sapphire substrate of which the principal plane is the C plane at a substrate temperature of 400° C., and subsequently an undoped gallium nitride (GaN) layer having a thickness of 1 μm is grown on the buffer layer at a raised temperature. The substrate temperature is then raised to 1010° C., and the superlattice layer is grown.
By adopting the above method, strain occurs between the AlGaN layer and the GaN layer, causing generation of an internal electric field. This makes the acceptor level of Mg shallow and thus improves the activation yield of the acceptor. Therefore, the p-type carrier density (hole density) increases, and this advantageously reduces the threshold current of the laser device.
In the second prior art (Japanese Laid-Open Patent Publication No. 8-97471), disclosed is a first contact layer made of highly doped p-type GaN that is in contact with an electrode made of nickel (Ni). The first contact layer has a thickness of 50 nm and a Mg density in the range of 1×1020 cm−3 to 1×1021 cm−3. This prior art discusses that with this construction, the contact resistance can be reduced, and also the operating voltage of the device can be lowered by attaining a high carrier density.
In the second prior art, if the first contact layer is doped with Mg at an excessively high density, the hole density contrarily becomes low. To overcome this problem, a second contact layer made of p-type GaN having a Mg density lower than the first contact layer is formed on the surface of the first contact layer opposite to the electrode. According to this prior art, the second contact layer is desirably doped with Mg at a density in the range of 1×1019 cm−3 to 5×1020 cm−3 for the purpose of increasing the hole density.
The prior art techniques described above have the following problems. In the first prior art, the superlattice structure of the p-type cladding layer is yet insufficient in attaining low resistance. In the second prior art, although the upper portion of the p-type contact layer is doped with the p-type dopant at a high density, this contrarily decreases the hole density.
In addition, the conventional doping techniques find difficulty in providing a steep impurity profile. In particular, when a p-type cap layer is formed on an active layer, for example, an especially steep impurity profile is required for suppression of diffusion of a p-type dopant to the active layer.